A radiotherapeutic apparatus is typically controlled by a Treatment Control Computer. When equipped with an Multi-Leaf Collimator (“MLC”) the Treatment Control Computer can be considered to contain a Radiation Control Computer which controls the radiation generation, an MLC Control computer which controls the shape of the MLC and a Gantry Control Computer which controls the position of the Gantry. These computers may physically be one or more computers but in this text are considered as distinct functional elements of the system.
“Mu” is an abbreviation for “monitor units”, which is the term used for units of radiation from the radiotherapeutic apparatus. An mu is equivalent to a unit of dose delivered to the patient under well defined calibration conditions. The relationship between mu and dose is modelled in the Treatment planning computer. The user interacts with the patient's prescription in units of dose but the Treatment planning computer defines the Treatment plan in units of mu. One of the tasks of a Treatment Planning computer is to ascertain the mu that need to be delivered by the apparatus in order to achieve a specific dose within the patient, both in terms of a sufficiently high dose in the tumour site and a sufficiently low dose in other parts of the patient. Informally, the use of the term ‘dose rate’ means ‘mu rate’
Intensity Modulated Radiotherapy is a generic term for a number of radiotherapy techniques that, essentially, vary the beam that is directed at the patient. That variation can be spatial, temporal, or both.
Known linac delivery technologies include the following.
Segmental or Static Multi-Leaf Collimator—“SMLC”—is where the Multi-Leaf Collimator (“MLC”) is static during irradiation. The MLC moves from one shape to the next in between irradiations. In one architecture, the point at which the irradiation stops and the MLC moves is controlled by the dosimetry hardware and Radiation Control computer. This results in exceptionally accurate delivery of dose per MLC shape. An alternative system uses a DMLC architecture to achieve the same effect. The MLC Control computer monitors the delivered dose and inhibits radiation when it detects it should move from one shape to the next. The inevitable control system delays associated with this architecture result in an uncertain dose per MLC shape and occasional missed shapes altogether.
Dynamic MLC—DMLC—is where the MLC moves during irradiation, with the gantry stationary. The MLC moves linearly from one shape to the next as a function of the delivered dose. The MLC control system has to monitor the delivered dose, and there is an inevitable delay. On older systems this delay was 200-300 ms, for more recent systems this is approximately 40 to 50 ms. This delay, together the response of the MLC, results in the shapes lagging behind the dose. This is extensively reported in the literature, but is widely regarded as not being clinically significant.
Rotational DMLC—RDMLC—is where the MLC moves during irradiation during a constant rotation of the gantry. The gantry moves at a constant mu per degree. The MLC moves linearly from one shape to the next as a function of the delivered dose. The shapes are usually, but not necessarily, defined at regular intervals around the arc. This can be achieved with a substantially independent MLC, Radiation and Gantry control computers.
Enhanced Rotational DMLC—ERDMLC—is where the MLC moves during irradiation during a rotation of the gantry and the gantry moves at a variable mu per degree. A variable gantry speed or variable dose rate (or both) can achieve the latter. Using variable dose rate alone has been analysed by the University of Gent as not being the preferred option as it gives longer delivery times. The MLC moves linearly from one shape to the next as a function of the delivered dose. The shapes and doses are usually, but not necessarily, defined at regular intervals around the arc. This technique requires a very high degree of integration between the MLC, Radiation and Gantry control computers and, to date, no linac has been able to deliver ERDMLC. At present, it is therefore a theoretical possibility only.
Treatment techniques involve a compatible treatment planning function and Linac delivery function, and known techniques are as follows:
Intensity Modulated Radiation Therapy—IMRT—is a sequence of MLC shapes with associated doses which can be delivered using SMLC and DMLC. The shapes are defined at a limited number of stationary gantry positions, typically 5 to 9. The shapes and doses are defined by an optimiser which attempts to meet objectives defined by the user. The treatment planning function is generally specific to the MLC constraints and the delivery technique.
Rotational Conformal Arc Treatments—RCAT—involves a constant rotation of the Gantry while the leaves are fitted dynamically to the projection of the target volume. This technique has been in use in Japan for many years. The delivery technique is RDMLC and only one arc is used.
Intensity Modulated Arc Therapy—IMAT—involves a treatment planning function in which the arcs and the positions of the leaves are not defined by the projection of the target volume but by an optimisation routine that tries to deliver the required dose distribution to the target and critical structures. In general a number of arcs are used over different ranges of gantry angles. The optimisation is like IMRT but includes the added flexibility of the rotational gantry. IMAT can be delivered via RDMLC, but this imposes a restriction on the optimisation of a constant mu per degree, which results in a sub-optimal plan. More ideally, the optimisation will be allowed complete freedom and an ERDMLC delivery technique will be used. The delivery times are exceptionally quick, typically 3 minutes for a complex plan. Superficially this technique looks the same as RCAT but the difference is how the MLC shapes are determined.
IMAT is discussed, for example, in Duthoy et al, “Clinical implementation of intensity-modulated arc therapy (IMAT) for rectal cancer”, International Journal of Radiation Oncology, Volume 60, Issue 3, 1 Nov. 2004, pp 794-806 which ends “We identified significant potential for improvements both at the levels of planning and delivery. The single most important technical improvement for IMAT is the implementation of a variable gantry speed”, i.e. an apparatus capable of ERDMLC.
Optimized Segment-Aperture Mono-Arc Therapy—OSAMAT—is a special class of IMAT in which only one arc is used. This seems suitable for some clinical indications. It could also be regarded as a refinement of RCAT. Similar to IMAT the delivery technique can be simply RDMLC but more ideally ERDMLC. The delivery times are exceptionally quick, typically 1 minute.
Arc Modulation Optimisation Algorithm—AMOA—is the technique used by 3D Line Medical Systems. The leaf shapes are defined by the anatomy (as in RCAT) and then the arcs are divided into smaller sub arcs of about 20 degrees and the weight or mu per degree of these sub arcs are optimised to give the best dose distribution (similar to IMAT or IMRT). Thus, this is a form of IMAT or OSAMAT in which the option of modifying the leaf positions is not used. This is very quick to plan and to deliver, especially using the ERDMLC delivery technique.
Helical Intensity Modulated Arc Therapy—HIMAT—is a development of the IMAT technique where the patient is translated longitudinally simultaneously with the gantry rotation. This effectively makes the longitudinal length of the treatable field unlimited and truly competes with a Tomotherapy delivery solution. U.S. Pat. No. 5,818,902 and WO97/13552 show details of this. This typically has an MLC in a fixed orientation with the leaves moving across the patient. The MLC can have high-resolution leaves and a limited field size, as the field size can be extended by use of the helical technique.
The delivery technique for HIMAT can be simply RDMLC as the multiple rotations will allow the flexibility of increased dose from certain angles. The delivery times are exceptionally quick, typically 3 minutes for a complex plan.